LumaSense Technologies M67 - Manual (Page 34)

M67, M67S Manual
33
9. Emissivity
The emissivity of an object is the ratio of the total thermal radiant energy intensity emitted by an object corresponding to the
value from a blackbody at the same temperature.
The emissivity of real objects is always higher that 0 and less than 1. Typical values at room temperatures are shown below:
MATERIAL TOTAL EMISSIVITY
Aluminum 0.05
Carbon 0.81
Cotton Cloth 0.77
Gold 0.02
Paper 0.75
Plaster 0.92
Sand 0.76
Silver 0.02
Wood 0.89
Bright and shiny metal materials have low emissivities whereas dull, opaque surfaces have high emissivities. Recall also that a
material may have a low emissivity because it has a higher transmissivity. Further, the emissivity of a material may vary with
temperature and wavelength.
For proper calibration, the emissivity of the target must be known accurately. Most practical values for emissivity lie between
0.1 and 0.99. Values below 0.1 do exist (as shown in the Table above for polished metals). However, at very low emissivities, a
substantial proportion of the thermal radiation received by the instrument might possibly come from reflections or other random
sources. This could result in temperature reading errors for targets of very low emissivity.
Tables of emissivity are available for a wide variety of materials. Fortunately, most materials have high emissivities,
approaching unity. Examples are most organic and non-metallic materials such as rubber, colored plastics, cloth, paint, wood,
and paper.
9.1 Emissivity calibration
When the emissivity is not known, it may be estimated by one of two methods described below. In either case, the
determination of emissivity should be made at or near the temperature of expected use; also, the emissivity calibration should
be checked periodically. The emissivity may be set in the INFRADUCER by adjusting pushbuttons located at the rear end of
the device. Note that the emissivity reading is shown as a percentage rather than a fraction. Thus, an emissivity of 0.72 will set
as 72 on the dial.
METHOD 1
This method can be used only if an accurate estimate of the target temperature is available from some independent source, for
example, a fine wire thermocouple or other direct surface temperature measuring device.
Read the surface temperature measurement with the INFRADUCER, with the emissivity dial set at 99. The instrument reading
should now be less than or equal to the known, true temperature. If not, no further adjustment is possible. Check immediately
for background radiation influence, reflections or other sources of error.
If the INFRADUCER reading is less than the true temperature, reduce the emissivity setting until a correct reading is obtained.
Record the emissivity for future reference.
METHOD 2
This method is applicable if the target is large enough to allow the INFRADUCER to be focused at two adjacent areas. Where
it should be possible to paint one of the target surface areas with a flat black paint (whose emissivity is known to be near 1.0).
Paint one of the two selected areas on the target with the paint. Set the INFRADUCER emissivity dial to 99. Measure the
painted target area temperature. This is the true temperature of the surface. Now, aim the INFRADUCER to the adjacent
unpainted area and adjust the emissivity control until the recorded temperature equals the previously determined true
temperature. The emissivity setting should be recorded for future reference.